Put the cauliflower in a large pot of boiling salted water,
and blanch for 2 to 3 minutes, or until just tender when pierced.
Drain the cauliflower, and set on a baking sheet. Roast in
the oven for 20 minutes. Do not allow it to color.

Heat the oil in a large pot or sauté pan, add the onion
and shallots, and sweat over very low heat until they start
to break down, at least 20 minutes. Again, do not allow them
to color. Add the roasted cauliflower to the pan, and season
with salt and pepper. Sauté for 3 minutes, again making
sure the vegetables don’t color. Add 2 cups of vegetable
stock, and cook until all the liquid has evaporated. Stir
in the grated horseradish and apple, and remove from the heat.

Heat the remaining vegetable stock in a saucepan. Put the
cauliﬂower mixture into a blender along with a little
hot stock, and purée, adding the remaining stock.

Return the soup to the pot, and place over low heat. Whisk
in 1 teaspoon of the sherry vinegar, or more to taste. Season
with salt and pepper. Serve immediately.